The related patent applications describe two-stroke, opposed-piston engines in which pairs of pistons move in opposition in the bores of ported cylinders. During a compression stroke, as two opposed pistons move toward each other in a cylinder bore, a combustion chamber is formed in the bore, between the end surfaces of the pistons. Fuel is injected directly into the volume of the combustion chamber when the pistons are at or near respective top center (“TC”) locations in the bore. When the end surfaces are closest to each other, near the end of the compression stroke, minimum combustion chamber volume (“minimum volume”) occurs. The fuel is injected through fuel injector nozzles positioned in diametrically-opposed openings through the sidewall of the cylinder. The fuel mixes with charge air admitted into the bore. As the air-fuel mixture is compressed between the piston end surfaces, the compressed air reaches temperature and pressure levels that cause the fuel to ignite; combustion follows. Combustion timing is frequently referenced to minimum volume. In some instances injection occurs at or near minimum volume; in other instances, injection may occur before minimum volume. In any case, in response to combustion the pistons reverse direction and undergo a power stroke. During the power stroke, the pistons move away from each other toward bottom center (“BC”) locations in the bore. As the pistons reciprocate between top and bottom center locations they open and close ports formed in respective intake and exhaust locations of the cylinder in timed sequences that control the flow of charge air into, and exhaust from, the cylinder.
In many aspects of piston constructions for two-stroke, opposed-piston engines it is desirable to utilize pistons with crowns having contoured end surfaces that interact with swirl in the cylinder and with squish flow from the periphery of the combustion chamber. The interaction produces complex, turbulent charge air motion that encourages air/fuel mixing. The related applications are directed to opposed-piston applications in which the piston end surfaces define combustion chambers having specific shapes that encourage turbulence. In these applications the combustion chamber is defined between opposed ridges that extend on opposite sides of a chamber centerline that runs between diametrically-opposed openings in the combustion chamber through which fuel is injected; thus the chamber centerline corresponds to a piston diameter D between the openings. In some instances, the ridges are symmetrically curved with respect to the chamber centerline in order to guide air flow and fuel plumes. In longitudinal cross-section, these combustion chambers have the shape of a non-looping simple closed curve centered on the centerline that decreases in area from a central portion toward either opening. At minimum volume, the symmetrical ridge shapes give the combustion chamber space an elongated, generally symmetrically shape in plan which has opposing curved sides and runs along the centerline. The widest portion of the combustion chamber occurs at or near the longitudinal axis of the cylinder (which is collinear with the longitudinal axes of the pistons and transverse to the chamber centerline). From there the combustion chamber space tapers symmetrically in opposing directions to the openings in the combustion chamber. This shape conforms to the configurations of the fuel plumes and guides them as they spread while travelling toward the central portion of the combustion chamber. See, for example, the ellipsoidal shape of the combustion chamber described in U.S. Pat. No. 8,800,528.
Combustion chamber symmetry may in some instances reduce combustion efficiency. The swirl component of charge air tends to urge the plumes of fuel toward respective ridges that define the sides of the combustion chamber, thereby reducing air utilization and hence thermal efficiency. Combustion chamber symmetry may also work against effective control of emissions if swirl pushes the plumes into contact with the sides of the combustion chamber, which can cause partial flame quenching and production of soot. Another possible drawback of symmetry can occur if the plumes ignite while in contact with the sides, which can result in increased heat transfer to the piston crown and risk of crown oxidation.